This invention relates to high power (nonlinear) testing of RF or microwave transistors (DUT) in the frequency and time domain for Noise and Load Pull measurements (see ref. 1). Load pull is the method by which the load impedance presented to the DUT at a given frequency is changed systematically and the DUT performance, together with the associated impedance, is registered, with the objective to find an optimum depending on the overall design objectives, see ref. 1. This objective may be maximum power, gain, efficiency, linearity or else. The same is valid for the source side of the DUT. Passive (slide screw) tuners are used to create the various impedances presented to the DUT, see ref. 2. The electrical signals injected into the input of the DUT and extracted from the output can be measured using sampling devices, such as signal couplers (FIG. 1), see ref. 3. At high power the (nonlinear) DUT is saturating and deforming the sinusoidal input signal. As a result part of the output power is contained in harmonic frequency components. The DUT performance can only be fully optimized when all frequency components (fundamental and harmonic) are properly impedance-matched. This requires independent harmonic tuning, mainly at the DUT output and at the DUT input.
In the case of noise measurements the tuners are used to generate arbitrary source impedances and appropriate software is then used to extract the noise parameters. In all cases the size (length) of the tuners at low frequencies is a mechanically limiting factor, since proper coverage of the Smith chart reflection factor area requires the reflective probes (slugs) of the slide-screw tuner to travel along the airline at least one half of a wave-length (λ/2) at the lowest frequency of operation; the wave-length λ is inverse proportional to the frequency F, following the rule λ[cm]=30/F[GHz]; i.e. at 1 GHz λ=30 cm, at 10 GHz λ=3 cm.
Harmonic impedance tuners have been introduced in 1999, see ref. 8, and 2004, see ref. 4 and 5. The early versions, see ref. 8, used resonant probes that allow tuning only at maximum Gamma (Γ≈1) and need to be replaced for changing frequencies; in many cases this is sufficient, but in general it is a limitation. Later introduced harmonic tuners, see ref. 4 and 5, allow frequency agility and full Smith chart coverage, but at the cost of higher mechanical complexity and linear size (length), FIG. 2. Their accuracy is equal to or better than previous versions. They comprise a number of independent wideband reflective RF probes (31) insertable into and movable horizontally inside the slot of a slotted low loss transmission airline (slabline) (32). To tune independently three frequencies, harmonic or not, it has been shown experimentally, that there is need for at least three such probes (31), see ref. 5, whereas for two frequencies only two probes are required, see ref. 9. It needs to be noted that there is no theoretical explanation why two probes handle two frequencies and three probes handle three frequencies. It is simply an experimental finding of which there is no proof of the contrary. Each probe is attached to the vertical axis of a carriage (33) and positioned by a precision remotely controlled gear mechanism, FIG. 2. The main shortcoming of such tuners is their linear length, due to the required length of the slabline, because it has been found experimentally that, in order to generate arbitrary reflection factors (impedances) at any harmonic or not frequency, each probe and associated carriage must move horizontally approximately at least one half of a wavelength (λ/2) at the lowest (fundamental) frequency Fo, FIG. 2. Mechanically speaking, the lowest fundamental frequency determines the length of the tuner as discussed previously.
In a real tuner apparatus (FIG. 2) the size of additional supporting items, such as the length (width) of the mobile carriages themselves (LC) and the length (thickness) of the side-walls (LW) of the tuner housing, add to the overall tuner length. In practical terms the minimum overall length of the slabline of a three carriage harmonic tuner, without the size of the input and output connectors, at the fundamental frequency Fo is: L=3*λ/2(Fo)+3*LC+2*LW (see FIG. 2). For example, considering a typical width (LC) of a carriage being LC≈3 cm and the thickness of each side-wall being LW≈1 cm, then a, prior art linear three-carriage tuner, (FIG. 2), starting at a lowest frequency of 400 MHz (0.4 GHz) has a minimum length of L≈3*15 cm/0.4+3*3 cm+2*1 cm=3*37.5 cm+9 cm+2 cm=123.5 cm.